The invention relates to an ultrasound process for the determination of the location of a local discontinuity of ultrasonic homogeneity forming a parietal surface situated in a tissue, including a step for the acquisition of a radio-frequency signal representing amplitudes of echoes which are returned by tissue scatterers as functions of their digital depths along an excitation line crossing the parietal surface, and are transmitted by a transducer coupled to the tissue and associated with an ultrasound apparatus. The invention also relates to an ultrasound process for the determination of the absolute value of the arterial radius in a tissue from ultrasound frequency signals propagating in the tissue. The invention furthermore relates to an ultrasound apparatus having means to carry out such processes.
The invention is used in the field of manufacture of ultrasound apparatus for clinical examination of arteries so as to assist medical diagnosis of arterial abnormalities without use of invasive means.
For detecting abnormalities of arteries, especially abnormalities of arterial walls, it is helpful to process the ultrasound radio-frequency signals provided by ultrasonic apparatus in order to determine arterial parameters such as the compliance of the arterial walls; this implies accurate determination of the arterial radius variations during a cardiac pulse and, for this purpose, accurate determination of the absolute value of the arterial radius.
An ultrasound signal processing method for determining arterial wall positions in a tissue is already known from the European Patent Application EP 0 840 139 A1. This known method consists in a segmentation of the amplitude signals derived from the ultrasound radio-frequency signals of an ultrasound transducer applied to a tissue region including an artery delimited by its walls. The segmented signals are subjected to a threshold operation in order to separate the pixels which are situated on an excitation line crossing the artery and belong to the internal parietal boundaries of the artery. The determination of the threshold value is based on the difference of gray level between the pixels of the lumen and the pixels of the walls of the artery at these internal parietal boundaries. After the threshold operation, the boundaries are subjected to a smoothing operation using morphological filters. Subsequently, the co-ordinates of the pixels situated on the resultant boundaries and the same excitation line are determined.
This method is used to construct a moving pattern which follows the arterial wall motions during a cardiac cycle in order to assist the diagnosis of abnormalities such as stenosis. As a matter of fact, stenosis or rigid plaques prevent the arterial walls from dilating normally under the influence of the blood pulse. These abnormalities can be diagnosed more easily from the images displayed using the known method, which images show amplified motions of the pattern of the boundaries represented incrusted in an intensity image and tuned to the cardiac cycle.
For this purpose, the known method does not need to provide extremely accurate values of boundary pixel co-ordinates.
Nowadays, diagnoses of vascular diseases and therapeutic choices have to be based on the analysis of the arterial lesion morphology and on the analysis of blood flow. Such analyse imply methods for accurate determination of the arterial wall compliance which require in advance very accurate determination of physiological parameters, and notably determination of extremely accurate values of the arterial radius.
It is an object of the invention to provide a process which can be carried out without invasive means in order to determine the location of a local discontinuity of ultrasonic homogeneity forming a parietal surface situated in a tissue. This object of the invention is achieved by means of an ultrasound process as claimed in claim 1.
It is another object of the invention to provide such a process for determining the location of an artery wall situated in a tissue. This object of the invention is achieved by means of an ultrasound process as claimed in claim 7.
It is another object of the invention to provide the absolute radius value of an artery in a tissue with great accuracy. The problem is that the arteries have mostly small diameters in comparison with other elements of the tissue, and a particular problem is that the wall motions during the cardiac cycle are extremely small and fugitive. So it is very difficult to measure accurately the arterial radius at each instant of the cardiac cycles in order to further determine the radius variations under the influence of blood flow.
This object of the invention is achieved and the problems are solved by means of a process as claimed in claim 8.
This process offers significant advantages because it can be carried out automatically and because it provides very accurate absolute values of the radius of an artery, at a given instant of the cardiac cycle.
An ultrasound apparatus as claimed in claim 9 is provided with a processing system for carrying out the above processes.
The advantage is that the arterial radius can be determined in vivo without using invasive means.